Determining throttle leakage at idle, or bad iac valve.

So my truck runs at 900rpm with the idle air control valve unplugged, but will not get below 1550 rpm with it plugged in.

My question is. Does this mean the ecu is keeping the rpm at 1500 for a reason? Or do i need a new iac valve.


My ecu reports the o2 sensor system is incomplete but does report the voltages from both sensors, and does run in closed loop, but it seems the ecu is trapped in a feed back loop where it cannot maintain a steady rpm regardless of throttle position between idle 900rpm and 2500rpm with the iac unplugged.

With the iac valve plugged in i can get acceptable throttle response and get the short term fuel trim down to near 0% by adjusting the throttle position sensor, but 1500rpm idle is unacceptable.


With the iac valve unplugged and a cold start. The engine runs in open loop for several minutes, pretty rich, and then goes into closed loop and will oscillate between 500 and 900 rpm until it continues to warm up.

 

Do you have a possible vacuum leak somewhere ? I had similar idle issues in my 98 and it was the throttle plate worn out and letting excess air get in. I sent my TB to max bore and cleared it all up and it idles very nice. Not saying that’s what it is but could be.

 

Have chased this myself. 1100rpm at op temp. Cleaned IAC which I believe was stuck open. It ended up working intermittently, believe the solenoid was sticking or failing, so rpm’s were low, stalling when below op temp. Replaced with aftermarket IAC that worked fine but had 900rpm at op temp. Aftermarket proved to have a different dimension air port. Replaced with OEM and consistent 700rpm and function. Through all that I cleaned entire intake, throttle body, plenum, EGR and plenum port, everything to the block. Installed a catch can and get about one to two spoon fulls of sludge at oil change cycles. Anything that calls for increased rpm could be culprit, power steering and brake booster but vac leaks and IAC seem to be more common issue from what I’ve read.
https://www.tacomaworld.com/threads/iac-tps-advice-96-2-7l-4wd-mt-143kmiles.780248/
https://www.tacomaworld.com/threads/throttle-body-iac-cleaning.819537/#post-29393091

 

Unfortunatly the guy at Maxbore was killed in a car accident ealier this year and the family has shut down the operation. Not sure what to do now for throttle body repair.

 

oh man! didn't know about that. Sad news to hear.

 

The throat on my 96 3rz throttle body with over 450k mi. was pretty worn. Maxbore closed, so I built up missing material with belzona 1321 (because that's what I had). I wanted to try tomei throttle coat but finding it not so easy. I think maybe they don't make it anymore. Now it idles 900 instead of 1500 or more. I think I could improve on that with a little more effort, someday. Anyhow, if all else fails for you maybe this is something you might try. Be mindful not to block the vacuum holes if you still have egr.

 

i tend to believe the 900 rpm is leakage past the throttle plate and nowhere else.


the question is:

if unplugging the IAC valve drops the rpm from 1500 to 900..

does that mean the ECU is calling for 1500 rpm or does it mean the ECU always holds the IAC valve slightly open at all times, and additional vacuum leaks are the cause of the high idle.

 

I am unsure if it's what the ECU calls for. I do not think that the ECU is your problem though. Might be easier to swap in a used IAC and see if it duplicates the same thing. Just get a junk yard one. Make sure it's clean of course and try it out.

 

Same exact issue with the iac plugged vs unplugged after a few months of driving it unplugged it started cutting off mid drive shop said the ecu had a short ordered a remanned one hoping that is the cause of the idle issue replaced iac, throttle body, smoked it for leaks nothing 2000 Tacoma 6cyl manual btw

 

So going from memory since it’s been a while. IAC is three position, closed, half open, fully open. With no dc voltage applied to silenoid it sits at half open. You can fact check me with a bench test, some wire and a car battery. Three prongs in the plug, ground and two hots which reverse polarity making solenoid turn one way or the other. You can also just pinch off the bypass hose that goes to the IAC with your fingers and hear the rpm drop.

As I stated in the thread I linked previously, aftermarket IAC I had experience with had a larger opening so ECM dumped more fuel and had 900 rpm at op temp. Put in OEM IAC and it’s 700 per spec for my 2.7L manual transmission. Aftermarket IACs also seem to lack what I think is a thermocoupler type spring. I believe it slowly winds/unwinds so the rpm’s gradually reduce as temp goes up. Aftermarket IAC is just a plastic affair so I think rpm’s drop at an interval when op temp is reached, not a gradual decline and more sudden.

Many here assume high idle beyond vac leaks is air leakage in throttle body since they see some daylight with flapper closed, dash pot off etc. They want to max bore and get it closed fully to cut air off, fix high idle. That may be the case on some high mileage trucks but that would be my last resort after eliminating all other potential problems. My throttle body has daylight showing and it ain’t from high mileage wear. They are designed that way imo. I guess what I am trying to say is it could be possible to get a bit of air into the intake somewhere else (aftermarket IAC or vac leak etc), see daylight around the flapper in the throttle and assume it’s the issue. Then max bore or whatever to close it off more, it reduce rpm and think you’ve fixed the issue that you think was a worn throttle body when it really wasn’t the real issue. Might be the more costly “fix”.

 

Here is the link describing the diff in oem vs. aftermarket IAC in case it’s useful here.
https://www.tacomaworld.com/threads...from-oem-to-aftermarket.789119/#post-29268980

 

Just tossing this out there
In my replacing injector thread, another member convinced me to RR injector cups bc his friend chased a vacuum leak for months…..was a cracked Inj. Cup

fits your statement “past TB…”
GL

 

In my case that’s exactly what happens close off/ pinch vacuum line to iac and the idle drops as well as in plug the iac but I tested both oem and new and they both did the same thing is it still possible to be caused by the difference in iac valves from aem to aftermarket? Could it be related to ecu having a short that may be causing it to open constantly?

 

I know for a fact it was difference between aftermarket and oem IAC in my idle chase. So yes to that question. Could yours be one of the other potential issues others have suggested? Yes. If rpm is high you got air getting in there that ain’t meant to be. My issues started with high idle of 1200 from stuck open oem IAC. New aftermarket got me down to 900ish after all new vac hoses, cleaned complete intake to block, checked everything for leaks etc. Lastly, I pony’d up and bought oem IAC and it purrs at 700rpm. It don’t take but a little more air to raise rpm’s.

 

The information posted in the last few messages are exactly the information I am looking for. At this point, there is no need for further investigation.

The ECU in my case, is calling for 1530 rpm and I know from prior drive cycles it will attempt to sit at 900 after some amount of driving, but then give up, because the engine dies as the rpm swings wildly. I've mentioned this in a couple of my prior posts. I suppose I could take the IAC valve apart and partially plug up the slot and see if it drops 900 to 800, and 1530 to 1400, but the ecu might throw a code, as it wouldn't be able to hit the rpm it wants to.

My post from may 9 shows a screen shot of the torque app, showing incomplete on a number of items that should be complete. no codes. except for the iac when I unplug it. Would be great if anyone with the torque app and a cheap adapter can confirm they should all say complete on a 1996 distributor era 2.7L, my 96 s10 shows complete on everything.

Since my may 9 post, I took the head off again to chase a blown head gasket (which may not have been blown, rather, its difficult to get all the air out of the coolant, and i had a bad heater valve which was a leak letting air in)

I lapped all the valves and swapped all the shims around until the clearances were a little tight but even across the board, I only had to grind a little off one of the valve stems so that I didn't have to grind the shim or buy a new one. since then, the 1800 rpm it idled at in prior posts has dropped to 1500 and it idles smoothly at 900 which it didn't before, and has acceptable but still terrible throttle response at 900 rpm idle with the IAC unplugged.

The valve guides and the valves looked suspiciously new and there was zero slop as if new. However some of the valves had a concave surface on the valve and the seats didn't look the greatest.. its as if the prior mechanic put new valve guides in, new valves.. but didn't cut the seats and just lapped the valves until they sealed.

I did such a job myself, on a 1.9L VW TDI. I had to chuck the valves back up in the lathe, cut the 3 angles.. then lap.. three times in a row for each valve, otherwise the valve, which is softer than the seat, wears concave and the contact patch becomes too wide.

my method of getting the TPS aligned correctly is to adjust it till the fuel trim reads close to 0 percent at idle. -this does seem to help the computer figure things out and improves throttle response.

but given the prior screen shot.. even though the ECU throws no codes and says its in closed loop (and it is, you can't smell gas out the exhaust).. its pretty clear something isn't working right.


I've been thinking recently about the poor throttle response.

Since the engine has pairs of injectors, and the V6 is even worse with two pairs of 3 injectors...

Half, or in the case of the v6, 2/3rds the gas is from the prior conditions that existed before the intake valve was opened!

so its always going to be lean as the throttle opens and rich on throttle closing.

If the ecu over compensates by dumping in too much fuel as the throttle opens, you could get so much fuel it doesn't burn because its too rich.

In my opinion helps explain the poor performance of aftermarket injectors... because the ecu is hardwired from the factory with the OEM fuel map for oem injectors and it doesn't contain an algorithm needed to correct for non oem injectors or malfunctioning pressure regulators.

 

Holy Crap

In your opinion (as I read)
AM injectors the cause?????

 

well you would think if the flow rate is mostly proportional to pressure (which stays the same) and the amount of fuel that is ejected from the injector is exactly proportional to the time that the solenoid is energized, then it wouldn't matter if the injectors are after market as long as the fuel is sufficiently atomized... would it?

This isn't rocket science.


yet so many say the aftermarket fuel injectors don't run right.

this is in spite of the fact that half of the gas evaporates on the hot intake valve anyways during the prior injection cycle.

so i can only conclude the ecu has a non linear map for OEM flow rates.

i know from experience my truck didn't like the aftermarket MAF sensor.. yet.. its not hard to make a linear air flow sensor. so my conclusion isn't that the non oem is bad. rather, the OEM's non linearity is programmed into the ecu.

 

As I have also fell into same AM injector hole……

Again good info posted!

 

In my case I replaced the ecu and the idle is almost too low now it’s edging on the bottom almost stalling but hasn’t stalled yet hope this helps you narrow it down

 

have you given it time and drive cycles to self learn?